1. Field of the Invention
The present invention relates to an electrochemical cell having a power generating function or an electricity accumulating function such as a non-aqueous electrolyte battery or an electric double layer capacitor, and more particularly to an electrochemical cell of chip type adapted for surface mounting.
2. Description of the Background Art
An electrochemical cell such as a non-aqueous electrolyte battery or an electric double layer capacitor has been employed as a backup power supply for a clock device or a semiconductor memory, a reserve power supply of an electronic apparatus constituted of a microcomputer, an IC memory etc., or a power supply of a control circuit for a solar clock or for motor driving. Recently, the electrochemical cell has been investigated also as a power supply for an electric automobile and an auxiliary power accumulating unit for an energy conversion/storage unit.
With an increase in the density of component mounting for realizing smaller, lighter and more functional devices, a smaller and thinner structure is also requested for the electrochemical cell which is used as a power source for such devices. In a prior electrochemical cell of coin or button type, a metal case 401 and a metal cover 402 constituting the outer casing of the cell as shown in FIG. 4 serve also as terminals for electrical connection with an external circuit. Since such metal case 401 and metal cover 402 constituting the terminals are positioned above and below, it is common to weld a lead tab terminal A 403 and a lead tab terminal B 404 respectively to the case and the cover in advance and to solder the lead tab terminals to a circuit board. Therefore, an increased number of components including such lead tab terminals results in an increase in the number of production steps, thus leading to an increased cost, and a size reduction has been difficult because spaces are necessary for the terminals on the cell and the board.
For this reason, it has been investigated, in a structure of an external member for housing the electrical double layer capacitor element of a surface mounting type, to contact metal terminal plates with an insulating ring-shaped frame across a solder material so as to close the aperture of the frame and to irradiate a portion where the outer frame and the metal terminal plates are opposed across the solder material with a laser beam thereby adjoining the outer frame and the terminal plates (for example cf. patent reference 1). Also, it has been investigated, in the aforementioned configuration, to utilize Ni or stainless steel having a thermal expansion coefficient and a thermal conductivity close to those of alumina for the metal terminal plates thereby preventing a thermal strain (for example cf. patent reference 1).
[Patent Reference 1] JP-A-08-339943 (Page 2, FIG. 2)
However, though the configuration in the patent reference 1 can increase an effective space in the battery in comparison with a configuration of coin type, it is still associated with a drawback of requiring lead tab terminals for the electrical connection with the external circuit, since metal terminal plates serving also as external cases are positioned above and below also in this case.
Also, even in case of employing Ni in the metal terminals, heat generated by the laser beam irradiation causes a thermal expansion and a contraction, because of the thermal expansion coefficient (13×10−6/° C.) of Ni, thereby generating a distortion in the sealed aperture portion, whereby a sufficiently tight sealing cannot be obtained. Therefore, an internal resistance, a capacitance and a prolonged reliability cannot be realized with a sufficient level.
In the non-aqueous electrolyte battery and the electric double layer capacitor, for adhering and sealing an external case with a sealing plate, there can be employed, for example, a method of utilizing an adhesive, a soldering method, a caulking method or a fusing method. On the other hand, since the non-aqueous electrolyte battery or the electric double layer capacitor contains an electrolyte liquid therein and a high voltage is applied to current collecting members, a highly hermetic sealing is required for preventing anodic or cathodic corrosion of the positive and negative current collecting members and ensuring prolonged reliability. In fact there has been a difficulty in selecting a material for the sealing plate and a sealing method, in order to satisfy battery charactgeristics and sealing characteristics. For example, in case of sealing by providing an adjoining material or a adhesive material such as a solder at the edge of the outer casing, placing a sealing plate on such material and heating at a temperature equal to or higher than the melting point of the solder or at a hardening temperature of the adhesive material, the electrolyte in the outer casing is heated and evaporates to the exterior whereby the capacity of the battery is lost and a sufficient sealing cannot be attained.
Also in case of executing a resistance welding of a caulked portion for a highly reliable sealing or a laser or beam welding which is a fusing method, utilizing a sealing plate of a material resistant to cathodic corrosion (such as aluminum, stainless steel or nickel), a welded part of the sealing plate and the edge portion of the outer case assumes a high temperature, thereby generating a mechanical distortion in the sealed part since the material of such sealing plate shows a large thermal expansion. Also by the heat at the welding, a thermal expansion of the electrolyte increases the internal pressure, thereby eventually leading to a liquid leakage from the distorted part of the sealing. In this manner there have been experienced drawbacks of a loss in the capacity or a loss in the prolonged reliability.